The present invention generally relates to sensors for determining concentration of particular gas in a gas mixture. More particularly, the present invention relates to improving accuracy of non-dispersive infrared (NDIR) sensors in a gaseous environment, such as air, subject to dynamic pressure variation.
The size of a typical NDIR sampling chamber is fixed and is open to the atmosphere so that air can move in and out. The number of air molecules in a given volume is affected by temperature and air pressure but not the concentration of a target gas such as CO2. At low pressures or high temperatures, there will be fewer air molecules in the sample chamber, so there will also be fewer CO2 molecules, even though the parts per million (ppm) of CO2 hasn't changed. Fewer CO2 molecules “fools” the sensor into thinking that the CO2 concentration is lower than it really is. At high pressures or low temperatures, there are more air molecules in the sample chamber and more CO2 molecules, even though the CO2 concentration hasn't changed. More
CO2 molecules “fools” the sensor into thinking that the CO2 concentration is higher than it really is. Therefore a CO2 sensor calibration will only be accurate at one temperature and one air pressure.
Typically, NDIR sensors are connected to output devices or displays with intervening pressure and temperature compensation devices. Such prior art pressure compensation devices employ correction factors based on the Ideal Gas Law as follows:ppm CO2 corrected=ppm CO2 measured*((Tmeasured*pref)/(pmeasured*Tref)  (1)
where pmeasured=Current pressure; Tref=reference temperature; Tmeasured=Current absolute temperature; and pref=reference Barometric Pressure.
This system for pressure and temperature compensation has been found to be effective when applied to variations of atmospheric conditions which occur at a relatively slow rate. For example, when atmospheric pressure and temperature may change over a period of 12 hours or more as weather conditions change.
However, when there is a rapid change of ambient pressure of the environment in which the NDIR sensor is employed, the prior-art system of compensation may not be fully effective. For example, if an NDIR sensor were employed in an aircraft, cabin pressure in the aircraft might change in a matter of minutes as the aircraft climbs or descends. Similarly, if an NDIR sensor were employed in a land-based vehicle, the sensor may be exposed to varying pressures as the vehicle travels up or down mountainous terrain. Under such dynamically varying conditions, compensation based only on application of the Ideal Gas Law may not be sufficient to assure accurate results from the NDIR sensor.
In a typical vehicular installation of an NDIR sensor, a diffusion filter may be placed over an opening of the sensor to prevent entry of contaminants into the sensor. With the passage of time, the diffusion filter may become dirty. As the diffusion filter become increasing dirty, the rate of air flow into the sensor may diminish. As a result, the sensor may become increasingly insensitive to dynamic variations of atmospheric pressure and overall accuracy of the sensor may be correspondingly diminished.
As can be seen, there is a need for an NDIR sensor that may provide accurate gas concentration data even when operating in an environment subject to dynamic pressure changes. More particularly, there is a need for a system of compensating for pressure and temperature changes when such a sensor is incorporated in a vehicle. Still further, there is a need to prevent sensor insensitivity arising from progressive clogging of diffusion filters of NDIR sensors.